Treatment of water to remove gas vacuolate cyanobacteria

ABSTRACT

The treatment of water to remove gas vacuolate cyanobacteria is effected by subjecting the water to a predetermined pressure by being pumped down a centrally located pipe in a bore-hole, which is steel lined. This causes the gas vesicles in the cyanobacteria to collapse. The treated water is then stored in a settling tank where the cyanobacteria sink to the bottom and can be removed along with any other sediment in the water. The method can be applied to decrease the amount of cyanobacteria with gas vesicles in a lake or other water impoundment. (FIG. 1)

This application is a continuation of application Ser. No. 07/435,706,filed Nov. 13, 1989, now abandoned; which is a continuation ofapplication Ser. No. 07/101,658, filed Sep. 28, 1987, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method of and apparatus for thetreatment of water to remove gas vacuolate cyanobacteria.

In some water treatment processes at present in operation, the water tobe treated before being supplied to the customer is first allowed tostand in settlement tanks where the sediment and foreign bodies in thewater taken from the reservoir or lake is allowed to settle to form asludge at the bottom.

However, in certain areas and at certain times of the year gas-vacuolatecyanobacteria otherwise known as blue-green algae can form in a lake orreservoir. These cyanobacteria cannot be removed in settlement tanksbecause they float upwards and on the surface of the water.

BACKGROUND THEORY

It is well known that these cyanobacteria float because they contain gasvesicles that is they embody microscopic gas filled structures whichensure that they float rather than sink.

The gas vesicles of cyanobacteria are hollow, cylindrical structureswith cone shaped ends. When a gas vesicle is subjected to a moderatepressure (up to 1 bar) it shows only a small volume change (shown to beabout 1 part in 650 per bar for gas vesicles of cyanobacterium Anabaenaflos-aquae in a report by A. E. Walsby in the Proceedings of the RoyalSociety of London, Volume 216, pages 355-368) but at a certain criticalpressure the structure collapses flat. A. E. Walsby, in a paper in theProceedings of the Royal Society of London, Volume 178, pages 301-326,showed that the average critical collapse pressure of gas vesicles inAnabaena flos-aquae varies from 4 bar to 8 bar with a mean value ofabout 6 bar. When the gas vesicle collapses the conical ends flatten tosectors of circles and pull away from the central cylinder, whichflattens to a rectangular envelope. The contained gas diffuses out ofthe structure and dissolves in the surrounding water as the gas vesiclecollapses.

When the gas vesicles inside cells of cyanobacteria are collapsed thecyanobacteria lose their means of buoyancy and sink. This is illustratedin FIG. 1 of a paper by A. E. Walsby in Bacteriological Reviews, Volume36 pages 1-32, an article which contains much other information on gasvesicles and their properties. It has been established from researchcarried out by P. K. Hayes and A. E. Walsby in a paper in the BritishPhycological Journal, Volume 21, pages 191-197 that the median criticalpressure of gas vesicles from different species of cyanobacteria variedfrom about 5-9 bar and were inversely correlated with the meandiameters.

PRIOR ART

Various methods of treating cyanobacteria or dispersing them have notbeen found to be successful and the most practical method of removalwould be to cause them to sink, by collapsing their gas vesicles,whereby they could be removed along with the rest of the sediment in thewater.

There have been two previous attempts .to do this. The first involved anattempt to collapse gas vesicles in cyanobacteria by subjecting them toultra sound as described in the discussion section of a paper by A. J.Brook in water treatment and examination, Volume 8 pages 133-137.

The second involved attempts to collapse gas vesicles by explosionsdetonated under water, described by A. E. Walsby in the New Scientist of21 Nov. 1968 pages 436-437, and by D. Menday in Water Research Volume 6,pages 279-284.

Neither of these methods have proved to be practicable in a method ofremoving cyanobacteria from water. We describe here a new method ofcollapsing gas vesicles of cyanobacteria with a hydrostatic head ofwater, and the subsequent removal of these pressure-treated organismsfrom water by sedimentation.

Although not directly relevant to the treatment of water to remove gasvacuolate cyanobacteria, a method for the disposal of solid wastes hasbeen disclosed in British Patent Specification No. 1,163,494. The methodfor disposing of solid biologically activated waste materials disclosedin this Specification, includes the steps of:

(a) providing the waste materials in an aqueous slurry;

(b) pumping the slurry to a well;

(c) transporting the slurry in the well to a porous and permeableunderground formation; and

(d) pressure injecting the slurry under positive well head pressuredirectly into the porous and permeable underground formation at apressure which the formation readily accepts the slurry withoutfracturing it.

Such a method would be totally unsuitable for the treatment of water toremove gas vacuolate cyanobacteria, in view of the following:

(a) the earlier Specification teaches the disposal of waste solids andnot purification of water; and

(b) the various steps proposed in the earlier Specification are costlyand time consuming and could in no way be technologically adapted to thetreatment of water.

Other methods not directly relevant to the treatment of water to removegas vacuolate cyanobacteria have been disclosed in British Patentspecifications 1,521,558; 1,557,731; 1,540,065; and 1,573,907. All ofthese relate to the treatment of waste water and sewage by injection ofgas containing free oxygen under pressure. The method employed in eachof these specifications is such that it would prevent the collapse ofthe gas vesicles in gas vacuolate cyanobacteria as described by A. E.Walsby in the paper in the Proceedings of the Royal Society Volume 178pages 301-326 and further, as is made clear in the last of thesespecifications No. 1,573,907, the addition of oxygen under pressurewould cause the material to float rather than to sediment out.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above problemsby providing a method of and apparatus for the removal of gas vacuolatecyanobacteria from water.

According to one aspect of the present invention there is provided amethod of treatment of water to remove gas vacuolate cyanobacteria whichincludes the steps of: hydrostatically subjecting the water removed froma body of water to a predetermined pressure in order to collapse the gasvesicles inside the cells of the cyanobacteria, and separating the thustreated cyanobacteria by allowing them to sink to the bottom ofsettlement means along with any other sediment in the water.

Preferably the said predetermined pressure is approximately six bar, butmay be as much as ten bar. A pressure exceeding 10 bar may be used butit is normally more than is needed to collapse the gas vesicles insidethe cells of cyanobacteria that live in fresh water.

According to another aspect of the present invention there is providedapparatus for treatment of water to remove gas vacuolate cyanobacteria,including: means for hydrostatically subjecting the water removed from abody of water to a predetermined pressure in order to collapse the gasvesicles inside the cells of the cyanobacteria; and settlement means forseparating the thus treated cyanobacteria that sink to the bottom ofsaid settlement means along with any other sediment in the water.

The means for applying a hydrostatic pressure to the water may comprisefirstly a lined bore-hole .sunk in the ground, or secondly a tower caselocated above the ground, or thirdly a pump delivery pipe having at itsremote end a diffuser or pressure relief valve necessary to ensure thatthe pump produces the required additional hydrostatic pressure in thepipe, or fourthly any combination of the bore-hole, the tower case andthe pump delivery pipe with diffuser or relief value. In the case of thebore-hole and tower, the water is peferably applied through a centrallylocated pipe extending down the lined bore-hole or tower case to thebottom thereof. The minimum depth of the bore-hole or height of thetower case lies in the range of 31 m to 102 m, the preferred depth orheight being 60 m.

Preferably the bore-hole is steel lined, but other designs of bore-hole,such as a down pipe linked to an up pipe in the form of a U-tube, may beused instead.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in greater detail by way ofexample with reference to the accompanying drawings, wherein;

FIG. 1 is a diagrammatic representation of one preferred form ofapparatus for the treatment of water to remove cyanobacteria;

FIG. 2 is an elevation view of one preferred apparatus forhydrostatically subjecting the water to a predetermined pressure inorder to collapse the gas vesicles in the cyanobacteria;

FIG. 3 is a diagrammatic representation of a first alternative form ofthe apparatus shown in FIG. 1; and

FIG. 4 is a diagrammatic representation of a second alternative form ofthe apparatus shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, the apparatus for the treatment of water toremove cyanobacteria comprises: a pump 10, a steel-lined bore hole 12and a settling tank 14. Water from a lake or reservoir 16 is pumped bythe pump 10 through a pipe 18 having an appropriate filter in its nozzleinlet section 20. The pumped water is passed to the bore-hole 12 via apipe 22. As mentioned above the bore-hole 12 is lined with a steellining to contain the water under pressure. A pipe 24 extends centrallydown the bore-hole 12 to within a short distance from the bottom. Anyother suitable lining material may be used instead of steel.

The preferred depth of the bore-hole 12 is about 60 m. However, theactual depth for any particular set of conditions is quite critical andhas to be determined by experiment. Sufficient pressure must begenerated, by the hydrostatic head at the bottom of pipe 24, to collapseenough gas vesicles to cause loss of the cyanobacteria's buoyancy. Theminimum pressure required depends on the species of cyanobacteriumpresent and may be from 3 bar to 10 bar. Since a pressure of 1 bar isgenerated by a vertical water column of 10.2 m the required depth ofpipe 24 below the surface is, correspondingly, about 31 m to 102 m.According to the nature of the cyanobacteria and the location, thepreferred depth may therefore lie in the range of from 31 m to 102 m.

The water from the reservoir 16 is thus pumped by the pump 10 down tothe bottom of the pipe 24, where for a depth of 60 m the hydrostaticpressure of the water at the bottom of the bore-hole 12 is approximately6 bar. The water which flows out of the top of the bore-hole 12 ispassed to the settling tank 14 by means of a pipe 26.

The construction of the bore-hole 12 is shown in greater detail in FIG.2. After the bore-hole 12 has been dug in the ground it is sealedagainst penetration of liquid from the ground by being steel lined witha series of plates 13 or annular rings which are welded together. Acircular steel plate 15 is provided at the bottom of the bore-hole 12.As shown, the pipe 22 from the pump 10 is connected to the centrallylocated vertical pipe 24 by means of an elbow joint 28 and flanges 30and 32. The pipe 24 is centrally located within the steel-linedbore-hole by means of locating spiders 34 which are provided atequi-spaced intervals from top to bottom of the bore-hole 12.

In operation, water from the lake or reservoir 16 containingcyanobacteria is pumped to the bottom of the bore-hole 12 by means ofthe pipe 24, where the hydrostatic pessure builds up to about 6 bar.This pressure is sufficient to collapse the gas vesicles in thecyanobacteria, so that when they are carried out of the bore-hole 12 andinto the settling tank 14 via the pipe 26, their specific gravity is nowgreater than unity so that they sink with the other sludge and foreignbodies to the bottom of the settling tank leaving clean treated water atthe top of the settling tank from which it can be drawn for furthertreatment before being supplied to the customer.

A first alternative construction is shown in FIG. 3, wherein the heightof the level of the lake or reservoir is above the location of thetreatment apparatus and part of the hydrostatic head can be supplied bythe difference in vertical level between the reservoir surface and thetop of the bore-hole or between the top of a tower 44 and the top of abore-hole 40. The required depth of the bore-hole 40 is reducedaccordingly. As shown in FIG. 3, water from a reservoir 16 is pumped upto the top of a tower 44 by means of the pump 10 through a pipe 46. Thebottom of the tower 44 is connected to the pipe 24 of the bore-hole 40by a pipe

Water is pumped into the top of the tower through the pipe 46 at a rateso as to maintain the level of water near the top of the tower 44. Inthis case the desired pressure of 6 bar can be produced by ensuring thatthe total head between the top of the tower 44 and the bottom of thepipe 24 is 60 m.

A second alternative construction is shown in FIG. 4 in which necessaryto collapse the gas vesicles inside the cells of the cyanobacteria canbe generated in a water container or pipe. As shown a pump 50 takeswater from the reservoir 16 via the pipe 18 and supplies it to ahorizontal cylindrical container or pipe 52 which is provided with adiffuser or pressure relief valve 54 at the other end. Water passingthrough the valve 54 is collected in the settling tank 14. The pressurewhich builds up in the container or pipe 52 is sufficient to collapsethe gas vesicles inside the cells of the cyanobacteria which then sinkto the bottom of the settling tank 14. Lastly a combination of theconstructions shown in FIG. 3 and 4 is possible.

Whilst the above constructions have been described in connection withwater treatment processes for public water supply customers, it will beappreciated that the same method and apparatus could be used to decreasethe amount of cyanobacteria that contain gas vesicles in natural lakesor other water impoundments, and thereby to improve the water quality.In this case the pressure-treated water is returned to the lake eitherdirectly from the bore-hole, in which case the cyanobacteria will settleout on the lake bottom, or after removing the cyanobacteria bysedimentation in a settling tank. The preferred method would be towithdraw water from a particular depth at one end of the lake and toreturn it after treatment to the lake at another depth at a point remotefrom the withdrawal site, so as to minimize mixing of the treated anduntreated water. Regard would be paid to the patterns of watercirculation in the lake so that withdrawal occurred at a site, such asat lee shores, where the cyanobacteria tended to accumulate.

What we claim is:
 1. An apparatus, comprising:a tank; water in saidtank, said water containing gas vacuolate cyanobacteria; pressurizingmeans for subjecting the water in said tank to a predetermined pressuresufficient to collapse gas vesicles inside cells of the gas vacuolatecyanobacteria; settlement means communicating with said pressurizingmeans for holding the previously pressurized water and allowing the thustreated cyanobacteria to sink to the bottom of said settlement means;withdrawal means communicating with said settlement means forwithdrawing water from above said cyanobacteria; said pressurizing meansbeing hydrostatic including pipe means passing through the water to thebottom of the tank and subjecting the water to a pressure determined bythe height of the water in the tank.
 2. Apparatus for the treatment ofwater to remove gas vacuolate cyanobacteria which contain gas residuesand float on the surface of the water, said apparatus comprising:pumpmeans for extracting the water to be treated from a reservoir; a steellined bore-hole sunk into the ground; a centrally located vertical pipecontained within said bore-hole, the lower end of which is open andlocated a short distance above the bottom of the bore-hole; first pipemeans for connecting the top of said centrally located pipe to theoutlet of said pump means, said bore-hole having a depth such that thenatural hydrostatic head of water contained within said centrallylocated pipe provides a predetermined pressure which is sufficient tocollapse the gas vesicles inside the cells of the cyanobacteriacontained in said water; wherein substantially total pressure applied tocollapse the gas vacuolate cyanobacteria comes from the bore-hole andthe natural hydrostatic head of water contained within said centrallylocated pipe, a settling tank located adjacent said bore-hole; andsecond pipe means connecting the top of the bore-hole with the settlingtank to convey water which has passed down the centrally located pipe tothe settling tank so that the collapsed gas vacuolate cyanobacteriacontained in said water conveyed to the settling tank settle at thebottom of the settling tank by sedimentation and an output of water fromthe settling tank is free of gas vacuolate cyanobacteria.
 3. Apparatusaccording to claim 2, wherein the depth of the bore-hole lies in therange of 31 m to 102 m.
 4. Apparatus according to claim 2, wherein saidpressure at the bottom of the bore-hole provided by the hydrostatic headof water is in the range of 3 to 10 bar.
 5. Apparatus according to claim2, wherein said pressure at the bottom of the bore-hole provided by thehydrostatic head of water is approximately 6 bar.
 6. Apparatus fortreatment of water to remove gas vacuolate cyanobacteria which containsgas vesicles and float on the surface of the water, said apparatuscomprising:pump means for extracting the water to be treated from areservoir; a tower having inlet means at the top connected to the outletof the pump means and outlet means at the bottom; a steel linedbore-hole sunk in the ground; a pipe extending vertically down thecentral zone of said bore-hole; a pipe means connecting the outlet meansof the tower to the top of the pipe extending down the bore-hole; outletmeans at the top of the bore-hole; and a settling tank connected toreceive water from the outlet means of the bore-hole; the combinedheight of the tower and depth of the bore-hole being such that thenatural hydrostatic head of water provided by said tower and bole-holeprovides a predetermined pressure which is sufficient to collapse thegas vesicles inside the cells of the cyanobacteria contained in saidwater, wherein substantially the total pressure applied to collapse thegas vacuolate cyanobacteria comes from the bore-hole and the naturalhydrostatic head of water contained within said centrally located pipe;whereby so that the collapsed gas vacuolate cyanobacteria contained inthe water conveyed to the settling tank settle at the bottom of thesettling tank by sedimentation, thus providing an output of water fromthe settling tank is free of gas vacuolate cyanobacteria.
 7. Anapparatus as in claim 6, wherein said head forming means includes acylinder having vertical walls.
 8. An apparatus as in claim 6, whereinsaid head forming means includes a bore-hole sunk into the ground andhaving a steel lining.
 9. An apparatus as in claim 8, wherein the depthof the bore-hole is in the range of 31 meters to 102 meters.
 10. Anapparatus as in claim 6, wherein said hydrostatic head produces apressure 3 to 10 bar.
 11. Apparatus according to claim 6, wherein saidhydrostatic head provided by the combined height of the tower and depthof the bore-hold is 60 m to provide a pressure of 6 bar.